I get 7ish/100km on highway, 9ish in the suburbs. That's in a 2.2L manual Camry, driving more or less normally.
"So, what wastes the most fuel?
I shall try to drive more frugrally.
I stick to main roads/motorways, so there isn't much stop/start going on. "
Generally speaking, lower speed is better for economy (70-90km/h is optimum for most cars), but not accelerating slowly to that speed. Engines are actually most fuel efficient under moderate/heavy load at lower engine speeds. They are at their least efficient at wide-open throttle (max load) as the engine protects itself from meltdown in these conditions by dumping extra fuel into the chambers. Engines are also relatively inefficient under light load conditions - when you are only pressing the accelerator a tiny bit, as the fuel energy is going more towards overcoming pumping losses in the engine than propelling you forward. That means if you drive a manual, accelerate fairly quickly (not fully) between gears, but keep the revs low. If you have an automatic, accelerate but try to keep the engine in as high a gear as possible.
Where safe, instead of maintaining a steady speed with light throttle (and therefore light engine load) 'pulse' up to a certain speed (using 50%-80% throttle) and then stick the transmission in neutral and let the car cruise along (better to do this iin a manual). When the car is starting to slow down, pulse up again and put it back in neutral (you can also turn the engine off during the glide stage). This technique helps increase fuel efficiency by reducing pumping losses, but can be dangerous and inconsiderate if in moderate traffic.
